[swift-evolution] [SR-119] AnySequence --> Any*Collection promotion

[Austin Zheng]

Hello all,

Currently there exist at least four 'type-erased' sequence/collection types: AnySequence, AnyForwardCollection, AnyBidirectionalCollection, and AnyRandomAccessCollection.

The three Any*Collection types can be conceptually arranged into a 'ladder', in which collections can be constructed unconditionally from collection types above themselves, and conditionally from collection types below themselves. I've put together a little ASCII image of this here: (https://gist.github.com/austinzheng/829425242bef1573b668 <https://gist.github.com/austinzheng/829425242bef1573b668>).

Note that AnySequence doesn't currently fit into this ladder: Any*Collection instances can't easily be constructed from AnySequences. SR-119 (https://bugs.swift.org/browse/SR-119 <https://bugs.swift.org/browse/SR-119>) aims to change that. As per that ticket, I propose that we add three promotion APIs, sketched out as following:

extension AnyForwardCollection {
    init?<Element>(_: AnySequence< Element>)
}

extension AnyBidirectionalCollection {
    init?<Element>(_: AnySequence< Element>)
}

extension AnyRandomAccessCollection {
    init?<Element>(_: AnySequence< Element>)
}

Like their Any*Collection --> Any*Collection antecedents, these initializers construct a new Any*Collection out of an existing AnySequence, but only if the underlying sequence is compatible, and without copying the underlying sequence.

I'll also have to examine SE-0014 (https://github.com/apple/swift-evolution/blob/master/proposals/0014-constrained-AnySequence.md <https://github.com/apple/swift-evolution/blob/master/proposals/0014-constrained-AnySequence.md>) more closely to see if it has any effects on this proposal.

I plan on submitting a formal proposal at some point (probably by the end of the coming week), but first wish to solicit feedback, opinions, concerns, etc. Thanks for your time.

Best,
Austin
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